Unfortunately, Unreal doesn't make a distinction between the different kinds of iOS devices (they're all CONSOLE_IPhone), so you're out of luck.

Also, when submitting an app to the Apple Store, you only have one build for all devices: you won't be able to do it manually by making two different builds.

This is probably for a very good reason (Apple trying to keep compatibility between device types and iterations?), so double check that you really want to make a distinction. And if it's really the case, yes, you'll have to use some heuristic of yours, such as the display resolution.